Statellite direct radio broadcast receiver for extracting a broadcast channel and service control header from time division multiplexed transmissions

ABSTRACT

A method of receiving one of a plurality of prime rate channels transmitted via downlink signals from a satellite comprises the step of demodulating downlink signals into a baseband time division multiplex bit stream comprising frames generated by the satellite. Each of the frames comprises a plurality of time slots, each of the time slots having a set of symbols. Each symbol in the set of symbols corresponding to a respective one of the prime rate channels occupies a similer symbol position in each of the time slots. The method further comprises the steps of locating the frames in the bit stream using a master frame preamble inserted therein by the satellite, and retrieving from the set of symbols in each of the time slots of at least one of the frames the symbols that correspond to one of the prime rate channels.

CROSS-REFERENCE TO RELATED APPLICATIONS

Related subject matter is disclosed and claimed in co-pending U.S.patent application Ser. No. 08/569,346, filed by S. Joseph Campanella onDec. 8, 1995; in co-pending U.S. Patent Application of Robert L.Johnstone and S. Joseph Campanella, filed on Nov. 5, 1996 and entitled"System for Providing Location-Specific Data to a User"; in co-pendingU.S. Patent Application of S. Joseph Campanella, filed on Nov. 5, 1996under Ser. No. 08/746,019 and entitled "Direct Radio Broadcast Receiverfor Providing Frame Synchronization and Correlation for Time DivisionMultiplexed Transmissions"; in co-pending U.S. Patent Application of S.Joseph Campanella, filed on Nov. 5, 1996 under Ser. No. 746,020 andentitled "System for Formatting Broadcast Data for SatelliteTransmission and Radio Reception"; in co-pending U.S. Patent Applicationof S. Joseph Campanella et al, filed on Nov. 5, 1996 under Ser. No.08/746,069 and entitled "System for Managing Space Segment Usage AmongBroadcast Service Providers"; in co-pending U.S. Patent Application ofS. Joseph Campanella, filed Nov. 5, 1996 under Ser. No. 08/746,070 andentitled "Satellite Payload Processing System for Switching UplinkSignals to Time Division Multiplexed Downlink Signals"; in co-pendingU.S. Patent Application of S. Joseph Campanella, filed on Nov. 5, 1996under Ser. No. 08/746,071 and entitled "Satellite Payload ProcessingSystem Using Polyphase Demultiplexing and Quadrature Phase Shift KeyingDemodulation" ; and in co-pending U.S. Patent Application of S. JosephCampanella et al, filed on Nov. 5, 1996 under Ser. No. 08/746,072 andentitled "Satellite Payload Processing System Providing On-Board RateAlignment"; all of said applications being expressly incorporated hereinby reference.

FIELD OF INVENTION

This invention relates to a radio receiver for receiving time divisionmultiplexed downlink carriers.

BACKGROUND OF THE INVENTION

There presently exists a population of over 4 billion people that aregenerally dissatisfied and underserved by the poor sound quality ofshort-wave radio broadcasts, or the coverage limitations of amplitudemodulation (AM) band and frequency modulation (FM) band terrestrialradio broadcast systems. This population is primarily located in Africa,Central and South America, and Asia. A need therefore exists for asatellite-based direct radio broadcast system to transmit signals suchas audio, data and images to low-cost consumer receivers.

A number of satellite communications networks have been developed forcommercial and military applications. These satellite communicationssystems, however, have not addressed the need to provide multiple,independent broadcast service providers with flexible and economicalaccess to a space segment, nor consumers'need to receive high qualityradio signals using low-cost consumer radio receiver units. A needtherefore exists for providing service providers with direct access to asatellite and choices as to the amount of space segment that's purchasedand used. In addition, a need exists for a low-cost radio receiver unitcapable of receiving time division multiplexed downlink bit stream.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, a method ofreceiving one of a plurality of prime rate channels transmitted viadownlink signals from a satellite comprises the step of demodulatingdownlink signals into a baseband time division multiplex bit streamcomprising frames generated by the satellite. Each of the framescomprises a plurality of time slots, each of the time slots having a setof symbols. Each symbol in the set of symbols corresponding to arespective one of the prime rate channels occupies a similer symbolposition in each of the time slots. The method further comprises thesteps of locating the frames in the bit stream using a master framepreamble inserted therein by the satellite, and retrieving from the setof symbols in each of the time slots of at least one of the frames thesymbols that correspond to one of the prime rate channels.

A BRIEF DESCRIPTION OF THE DRAWING

These and other features and advantages of the present invention will bemore readily comprehended from the following detailed description whenread in connection with the appended drawings, which form a part of thisoriginal disclosure, and wherein:

FIG. 1 is a schematic diagram of a satellite direct broadcast systemconstructed in accordance with an embodiment of the present invention;

FIG. 2 is a flow chart depicting the sequence of operations forend-to-end signal processing in the system depicted in FIG. 1 inaccordance with an embodiment of the present invention;

FIG. 3 is a schematic block diagram of a broadcast earth stationconstructed in accordance with an embodiment of the present invention;

FIG. 4 is a schematic diagram illustrating broadcast segmentmultiplexing in accordance with an embodiment of the present invention;

FIG. 5 is a schematic block diagram of an on-board processing payloadfor a satellite in accordance with an embodiment of the presentinvention;

FIG. 6 is a schematic diagram illustrating on-board satellitedemultiplexing and demodulation processing in accordance with anembodiment of the present invention;

FIG. 7 is a schematic diagram illustrating on-board satellite ratealignment processing in accordance with an embodiment of the presentinvention;

FIG. 8 is a schematic diagram illustrating on-board satellite switchingand time division multiplexing operations in accordance with anembodiment of the present invention;

FIG. 9 is a schematic block diagram of a radio receiver for use in thesystem depicted in FIG. 1 and constructed in accordance with anembodiment of the present invention;

FIG. 10 is a schematic diagram illustrating receiver synchronization anddemultiplexing operations in accordance with an embodiment of thepresent invention;

FIG. 11 is a schematic diagram illustrating synchronization andmultiplexing operations for recovering coded broadcast channels at areceiver in accordance with an embodiment of the present invention; and

FIG. 12 is a schematic diagram of a system for managing satellite andbroadcast stations in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Overview

In accordance with the present invention, a satellitebased radiobroadcast system 10 is provided to broadcast programs via a satellite 25from a number of different broadcast stations 23a and 23b (hereinafterreferred to generally as 23), as shown in FIG. 1. Users are providedwith radio receivers, indicated generally at 29, which are designed toreceive one or more time division multiplexed (TDM) L-band carriers 27downlinked from the satellite 25 that are modulated at 1.86 Megasymbolsper second (Msym/s). The user radios 29 are designed to demodulate anddemultiplex the TDM carrier to recover bits that constitute the digitalinformation content or program transmitted on broadcast channels fromthe broadcast stations 23. In accordance with an embodiment of theinvention, the broadcast stations 23 and the satellite 25 are configuredto format uplink and downlink signals to allow for improved reception ofbroadcast programs using relatively low cost radio receivers. A radioreceiver can be a mobile unit 29a mounted in a transportation vehicle,for example, a hand-held unit 28b or a processing terminal 29c with adisplay.

Although only one satellite 25 is shown in FIG. 1 for illustrativepurposes, the system 10 preferably comprises three geostationarysatellites 25a, 25b and 25c (FIG. 12) configured to use frequency bandsof 1467 to 1492 Megahertz (MHz) which has been allocated forbroadcasting satellite service (BSS) direct audio broadcast (DAB). Thebroadcast stations 23 preferably use feeder uplinks 21 in the X-band,that is from 7050 to 7075 Mhz. Each satellite 25 is preferablyconfigured to operate three downlink spot beams indicated at 31a, 31band 31c. Each beam covers approximately 14 million square kilometerswithin power distribution contours that are four decibels (dB) down frombeam center and 28 million square kilometers within contours that areeight dB down. The beam center margin can be 14 dB based on a receivergain-to-temperature ratio of -13 dB/K.

With continued reference to FIG. 1, the uplink signals 21 generated fromthe broadcast stations 23 are modulated in frequency division multipleaccess (FDMA) channels from the ground stations 23 which are preferablylocated within the terrestrial visibility of the satellite 25. Eachbroadcast station 23 preferably has the ability to uplink directly fromits own facilities to one of the satellites and to place one or more 16kilobit per second (kbps) prime rate increments on a single carrier. Useof FDMA channels for uplink allows for a significant amount offlexibility for sharing the space segment among multiple independentbroadcast stations 23 and significantly reduces the power and hence thecost of the uplink earth stations 23. Prime rate increments (PRIs) of 16kilobits per second (kbps) are preferably the most fundamental buildingblock or rudimentary unit used in the system 10 for channel size and canbe combined to achieve higher bit rates. For example, PRIs can becombined to create program channels with bit rates up to 128 kbps fornear compact disc quality sound or multimedia broadcast programscomprising image data, for example.

Conversion between uplink FDMA channels and downlink multiple channelper carrier/time division multiplex (MCPC/TDM) channels is achievedon-board each satellite 25 at the baseband level. As will be describedin further detail below, prime rate channels transmitted by a broadcaststation 23 are demultiplexed at the satellite 25 into individual 16 kbpsbaseband signals. The individual channels are then routed to one or moreof the downlink beams 31a, 31b and 31c, each of which is a single TDMstream per carrier signal. This baseband processing provides a highlevel of channel control in terms of uplink frequency allocation andchannel routing between uplink FDMA and downlink TDM signals.

The end-to-end signal processing that occurs in the system 10 isdescribed with reference to FIG. 2. The system components responsiblefor the end-to-end signal processing is described in further detailbelow with reference to FIGS. 3-11. As shown in FIG. 2, audio signalsfrom an audio source, for example, at a broadcast station 23, arepreferably coded using MPEG 2.5 Layer 3 coding (block 26). The digitalinformation assembled by a broadcast service provider at a broadcaststation 23 is preferably formatted in 16 kbps increments or PRIs where nis the number of PRIs purchased by the service provider (i.e., n×16kbps). The digital information is then formatted into a broadcastchannel frame having a service control header (SCH) (block 28),described in further detail below. A periodic frame in the system 10preferably has a period duration of 432 milliseconds (ms). Each frame ispreferably assigned n×224 bits for the SCH such that the bit ratebecomes approximately n×16.519 kbps. Each frame is next scrambled byaddition of a pseudo random bit stream to the SCH. Information controlof the scrambling pattern by a key permits encryption. The bits in aframe are subsequently coded for forward error correction (FEC)protection using preferably two concatenated coding methods such as theReed Solomon method, followed by interleaving, and then convolutioncoding (e.g., trellis convolution coding described by Viterbi) (block30). The coded bits in each frame corresponding to each PRI aresubsequently subdivided or demultiplexed into n parallel prime ratechannels (PRCs) (block 32). To implement recovery of each PRC, a PRCsynchronization header is provided. Each of the n PRCs is nextdifferentially encoded and then modulated using, for example, quadraturephase shift keying modulation onto an intermediate frequency (IF)carrier frequency (block 34). The n PRC IF carrier frequenciesconstituting the broadcast channel of a broadcast station 23 isconverted to the X-band for transmission to the satellite 25, asindicated by the arrow 36.

The carriers from the broadcast stations 23 are single channel percarrier/frequency division multiple access (SCPC/FDMA) carriers.On-board each satellite 25, the SCPC/FDMA carriers are received,demultiplexed and demodulated to recover the PRC carriers (block 38).The PRC digital baseband channels recovered by the satellite 25 aresubjected to a rate alignment function to compensate for clock ratedifferences between the on-board satellite clock and that of the PRCcarriers received at the satellite (block 40). The demultiplexed anddemodulated digital streams obtained from the PRCs are provided to TDMframe assemblers using routing and switching components. The PRC digitalstreams are routed from demultiplexing and demodulating equipmenton-board the satellite 25 to the TDMA frame assemblers in accordancewith a switching sequence unit on-board the satellite that is controlledfrom an earth station via a command link (e.g., a satellite controlcenter 236 in FIG. 12 for each operating region). Three TDM carriers arecreated which correspond to each of the three satellite beams 31a, 31band 31c (block 42). The three TDM carriers are up converted to L-bandfrequencies following QPSK modulation, as indicated by arrow 44. Radioreceivers 29 are configured to receive any of the three TDM carriers andto demodulate the received carrier (block 46). The radio receivers 29are designed to synchronize a TDM bit stream using a master framepreamble provided during on-board satellite processing (block 48). PRCsare demultiplexed from the TDM frame using a Time Slot Control Channel(TSCC), as well. The digital streams are then remultiplexed into theFEC-coded PRC format described above with reference to block 30 (block50). The FEC processing preferably includes decoding using a Viterbitrellis decoder, for example, deinterleaving, and then Reed Solomondecoding to recover the original broadcast channel comprising n×16 kbpschannel and the SCH. The n×16 kbps segment of the broadcast channel issupplied to an MPEG 2.5 Layer 3 source decoder for conversion back toaudio. In accordance with the present invention, the audio output isavailable via a very low cost broadcast radio receiver 27 due to theprocessing and TDM formating described above in connection with thebroadcast station(s) 23 and the satellite 25 (block 52).

Unlink Multiplexing and Modulation

Signal processing to convert data streams from one or more broadcaststations 23 into parallel streams for transmission to a satellite 25will now be described with reference to FIG. 3. For illustrativepurposes, four sources 60, 64, 68, and 72 of program information areshown. Two sources 60 and 64, or 68 and 72, are coded and transmittedtogether as part of a single program or service. The coding of theprogram comprising combined audio sources 60 and 64 will be described.The signal processing of the program comprising digital information fromsources 68 and 72 is identical.

As stated previously, broadcast stations 23 assemble information fromone or more sources 60 and 64 for a particular program into broadcastchannels characterized by increments of 16 kbps. These increments arereferred to as prime rate increments or PRIs. Thus, the bit rate carriedin a broadcast channel is n×16 kbps were n is the number of PRIs used bythat particular broadcast service provider. In addition, each 16 kbpsPRI can be further divided into two 8 kbps segments which are routed orswitched together through the system 10. The segments provide amechanism for carrying two different service items in the same PRI suchas a data stream with low bit rate speech signals, or two low bit ratespeech channels for two respective languages, and so on. The number ofPRIs are preferably predetermined, that is, set in accordance withprogram code. The number n, however, is not a physical limitation of thesystem 10. The value of n is generally set on the basis of businessconcerns such as the cost of a single broadcast channel and thewillingness of the service providers to pay. In FIG. 3, n for the firstbroadcast channel 59 for sources 60 and 64 is equal to 4. The value of nfor the broadcast channel 67 for sources 68 and 72 is set to 6 in theillustrated embodiment.

As shown in FIG. 3, more than one broadcast service provider can haveaccess to a single broadcast station 23. For example, a first serviceprovider generates broadcast channel 59, while a second service providercan generate broadcast channel 67. The signal processing describedherein and in accordance with the present invention allows data streamsfrom several broadcast service providers to be broadcast to a satellitein parallel streams which reduces the cost of broadcasting for theservice providers and maximizes use of the space segment. By maximizingefficiency of space segment usage, the broadcast stations 23 can beimplemented less expensively using less power-consuming components. Forexample, the antenna at the broadcast station 23 can be very smallaperture terminal (VSAT) antenna. The payload on the satellite requiresless memory, less processing capability and therefore fewer powersources which reduces payload weight.

A broadcast channel 59 or 67 is characterized by a frame 100 having aperiod duration of 432 ms, as shown in FIG. 4. This period duration isselected to facilitate use of the MPEG source coder described below;however, the frame paired in the system 10 can be set to a differentpredetermined value. If the period duration is 432 ms, then each 16 kbpsPRI requires 16,000×0.432 seconds=6912 bits per frame. As shown in Fig.4, a broadcast channel therefore consists of a value n of these 16 kbpsPRIs which are carried as a group in the frame 100. As will be describedbelow, these bits are scrambled to enhance demodulation at the radioreceivers 29. The scrambling operation also provides a mechanism forencrypting the service at the option of the service provider. Each frame100 is assigned n×224 bits which correspond to a service control header(SCH), resulting in a total of n×7136 bits per frame and a bit rate ofn×(16,518+1427) bits per second. The purpose of the SCH is to send datato each of the radio receivers 29 tuned to receive the broadcast channel59 or 67 in order to control reception modes for various multimediaservices, to display data and images, to send key information fordecryption, to address a specific receiver, among other features.

With continued reference to FIG. 3, the sources 60 and 64 are codedusing, for example, MPEG 2.5 Layer 3 coders 62 and 66, respectively. Thetwo sources are subsequently added via a combiner 76 and then processedusing a processor at the broadcast station 23 to provide the codedsignals in periodic frames of 432 ms, that is, n×7136 bits per frameincluding the SCH, as indicated by processing module 78 in FIG. 3. Theblocks indicated at the broadcast station in FIG. 3 correspond toprogrammed modules performed by a processor and associated hardware suchas digital memory and coder circuits. The bits in the frame 100 aresubsequently coded for FEC protection using digital signal processing(DSP) software, application specific integrated circuits (ASICs) andcustom large-scale integration (LSI) chips for the two concatenatedcoding methods. First, a Reed Solomon coder 80a is provided to produce255 bits for every 223 bits entering the coder. The bits in the frame100 are then reordered according to a known interleaving scheme, asindicated by reference number 80b. The interleaving coding providesfurther protection against bursts of error encountered in a transmissionsince this method spreads damaged bits over several channels. Withcontinued reference to processing module 80, a known convolution codingscheme of constraint length 7 is applied using a Viterbi coder 80c. TheViterbi coder 83c produces two output bits for every input bit,producing as a net result 16320 FEC-coded bits per frame for eachincrement of 6912 bits per frame applied in the broadcast channel 59.Thus, each FEC-coded broadcast channel (e.g., channel 59 or 67)comprises n×16320 bits of information which have been coded, reorderedand coded again such that the original broadcast 16 kbps PRIs are nolonger identifiable. The FECcoded bits, however, are organized in termsof the original 432 ms frame structure. The overall coding rate forerror protection is (255/223)×2 =2+64/223.

With continued reference to FIG. 3, the n×16320 bits of the FEC-codedbroadcast channel frame is subsequently subdivided or demultiplexedusing a channel distributor 82 into n parallel prime rate channels(PRCs), each carrying 16320 bits in terms of sets of 8160 two-bitsymbols. This process is further illustrated in FIG. 4. The broadcastchannel 59 is shown which is characterized by a 432 ms frame 100 havingan SCH 102. The remaining portion 104 of the frame consists of n 16 kbpsPRIs which corresponds to 6912 bits per frame for each of the n PRIs.The FEC-coded broadcast channel 106 is attained following concatenatedReed Solomon 255/223, interleaving and FEC 1/2 convolution codingdescribed above in connection with module 80. As stated previously, theFEC-coded broadcast channel frame 106 comprises n×16320 bits whichcorrespond to 8160 sets of two-bit symbols, with each symbol beingdesignated by a reference numeral 108 for illustrated purposes. Inaccordance with the present invention, the symbols are assigned acrossthe PRCs 110 in the manner shown in FIG. 4. Thus, the symbols will bespread on the basis of time and frequency which further reduces errorsat the radio receiver caused by interference in transmission. Theservice provider for broadcast channel 59 has purchased four PRCs forillustrative purposes, whereas the service provider for broadcastchannel 67 has purchased six PRCs for illustrative purposes. FIG. 4illustrates the first broadcast channel 59 and the assignment of symbols114 across the n=4 PRCs 110a, 110b, 110c and 110d, respectively. Toimplement recovery of each two-bit symbol 114 set at the receiver, a PRCsynchronization header or preamble 112a, 112b, 112c and 112d,respectively, is placed in front of each PRC. The PRC synchronizationheader (hereinafter generally referred to using reference numeral 112)contains 48 symbols. The PRC synchronization header 112 is placed infront of each group of 8160 symbols, thereby increasing the number ofsymbols per 432 ms frame to 8208 symbols. Accordingly, the symbol ratebecomes 8208/0.432 which equals 19,000 kilosymbols per second (ksym/s)for each PRC 110. The 48 symbol PRC preamble 112 is used essentially forsynchronization of the radio receiver PRC clock to recover the symbolsfrom the downlink satellite transmission 27. At the on-board processor116, the PRC preamble is used to absorb timing differences between thesymbol rates of arriving uplink signals and that used on-board to switchthe signals and assemble the downlink TDM streams. This is done byadding, subtracting a "0" or neither to each 48 symbol PRC in the ratealignment process used on-board the satellite. Thus, the PRC preamblescarried on the TDM downlink has 47, 48 or 49 symbols as determined bythe rate alignment process. As shown in FIG. 4, symbols 114 are assignedto consecutive PRCs in a round-robin fashion such that symbol 1 isassigned to PRC 110a, symbol 2 is assigned to PRC 110b, symbol 3 isassigned to PRC 110c, symbol 4 is assigned to PRC 110d, symbol 5 isassigned to PRC 110e, and so on. This PRC demultiplexing process isperformed by a processor at the broadcast station 23 and is representedin FIG. 3 as the channel distribution (DEMUX) module 82.

The PRC channel preambles are assigned to mark the beginning of the PRCframes 110a, 110b, 110cand 110d for broadcast channel 59 using thepreamble module 84 and adder module 85. The n PRCs are subsequentlydifferentially encoded and then QPSK modulated onto an IF carrierfrequency using a bank of QPSK modulators 86 as shown in FIG. 3. Four ofthe QPSK modulators 86a, 86b, 86c and 86d are used for respective PRCs110a, 110b, 110c and 111d for broadcast channel 59. Accordingly, thereare four PRC IF carrier frequencies constituting the broadcast channel59. Each of the four carrier frequencies is up-converted to its assignedfrequency location in the X-band using an up-converter 88 fortransmission to the satellite 25. The up-converted PRCs are subsequentlytransmitted through an amplifier 90 to the antenna (e.g., a VSAT) 92aand 92b.

In accordance with the present invention, the transmission methodemployed at a broadcast station 23 incorporates a multiplicity of nSingle Channel Per Carrier, Frequency Division Multiple Access(SCPC/FDMA) carriers into the uplink signal 21. These SCPC/FDMA carriersare spaced on a grid of center frequencies which are preferablyseparated by 38,000 Hertz (Hz) from one another and are organized ingroups of 48 contiguous center frequencies or carrier channels.Organization of these groups of 48 carrier channels is useful to preparefor demultiplexing and demodulation processing conducted on-board thesatellite 25. The various groups of 48 carrier channels are notnecessarily contiguous to one another. The carriers associated with aparticular broadcast channel (i.e., channel 59 or 67) are notnecessarily contiguous within a group of 48 carrier channels and neednot be assigned in the same group of 48 carrier channels. Thetransmission method described in connection with FIGS. 3 and 4 thereforeallows for flexibility in choosing frequency locations and optimizes theability to fill the available frequency spectrum and to avoidinterference with other users sharing the same radio frequency spectrum.

The system 10 is advantageous because it provides a common base ofcapacity incrementation for a multiplicity of broadcast companies orservice providers whereby broadcast channels of various bit rates can beconstructed with relative ease and transmitted to a receiver 29. Typicalbroadcast channel increments or PRIs are preferably 16, 32, 48, 64, 80,96, 112 and 128 kbps. The broadcast channels of various bit rates areinterpreted with relative ease by the radio's receiver due to theprocessing described in connection with FIG. 4. The size and cost of abroadcast station can therefore be designed to fit the capacityrequirements and financial resource limitations of a broadcast company.A broadcast company of meager financial means can install a small VSATterminal requiring a relatively small amount of power to broadcast a 16kbps service to its country that is sufficient to carry voice and musichaving quality far better than that of short-wave radio. On the otherhand, a sophisticated broadcast company of substantial financial meanscan broadcast FM stereo quality with a slightly larger antenna and morepower at 64 kbps and, with further increases in capacity, broadcast nearcompact disc (CD) stereo quality at 96 kbps and full CD stereo qualityat 128 kbps.

The frame size, SCH size, preamble size and PRC length described inconnection with FIG. 4 are used to realize a number of advantages;however, the broadcast station processing described in connection withFIGS. 3 and 4 is not limited to these values. The frame period of 432 msis convenient when using an MPEG source coder (e.g., coder 62 or 66).The 224 bits for each SCH 102 is selected to facilitate FEC coding. The48 symbol PRC preamble 112 is selected to achieve 8208 symbols per PRC110 to achieve 19,000 ksym/s for each PRC for a simplifiedimplementation of multiplexing and demultiplexing on-board the satellite25, as described in future detail below. Defining symbols to comprisetwo-bits is convenient for QPSK modulation (i.e., 2² =4). To illustratefurther, if phase shift key modulation at the broadcast station 23 useseight phases as opposed to four phases, then a symbol defined as havingthree bits would be more convenient since each combination of three bits(i.e., 2³) can correspond to one of the eight phases.

Software can be provided at a broadcast station 23 or, if more than onebroadcast station exists in the system 10, a regional broadcast controlfacility (RBCF) 238 (FIG. 12) to assign space segment channel routingvia a mission control center (MCC) 240, a satellite control center (SCC)236 and a broadcast control center (BCC) 244. The software optimizes useof the uplink spectrum by assigning PRC carrier channels 110 whereverspace is available in the 48 channel groups. For example, a broadcaststation may wish to broadcast a 64 kbps service on four PRC carriers.Due to current spectrum use, the four carriers may not be available incontiguous locations, but rather only in non-contiguous locations withina group of 48 carriers. Further, the RBCF 238 using its MCC and SCC mayassign the PRCs to noncontiguous locations among different 48 channelgroups. The MCC and SCC software at the RBCF 238 or a single broadcaststation 23 can relocate PRC carriers of a particular broadcast serviceto other frequencies to avoid deliberate (i.e., jamming) or accidentalinterference on specific carrier locations. A current embodiment of thesystem has three RBCFS, one for each of the three regional satellites.Additional satellites can be controlled by one of these threefacilities.

As will be described in further detail below in connection with on-boardsatellite processing in FIG. 6, an on-board digitally implementedpolyphase processor is used for on-board signal regeneration and digitalbaseband recovery of the symbols 114 transmitted in the PRCs. The use ofgroups of 48 carriers spaced on center frequencies separated by 38,000Hz facilitates processing by the polyphase processor. The softwareavailable at the broadcast station 23 or RBCF 238 can performdefragging, that is, defragmentation processing to optimize PRC 110assignments to uplink carrier channels, that is, groups of 48 carrierchannels. The principal behind defragmentation of uplink carrierfrequency assignments is not unlike known software for reorganizingfiles on a computer hard drive which, over time, have been saved in sucha piece-meal manner as to be inefficient for data storage. The BCCfunctions at the RBCF allows the RBCF to remotely monitor and controlbroadcast stations to assure their operation wthin assigned tolerances.

Satellite Payload Processing

The baseband recovery on the satellite is important for accomplishingon-board switching and routing and assembly of TDM downlink carriers,each having 96 PRCs. The TDM carriers are amplified on-board thesatellite 25 using single-carrier-per-traveling-wave-tube operation. Thesatellite 25 preferably comprises eight on-board baseband processors;however, only one processor 116 is shown. Preferably only six of theeight processors are used at a time, the remainder providing redundancyin event of failures and to command them to cease transmission ifcircumstances require such. A single processor 116 is described inconnection with FIGS. 6 and 7. It is to be understood that identicalcomponents are preferably provided for each of the other sevenprocessors 116. With reference to FIG. 5, the coded PRC uplink carriers21 are received at the satellite 25 by an X-band receiver 120. Theoverall uplink capacity is preferably between 288 and 384 PRC uplinkchannels of 16 kbps each (i.e., 6×48 carriers if six processors 116 areused, or 8×48 carriers if all eight processors 116 are used). As will bedescribed in further detail below, 96 PRCs are selected and multiplexedfor transmission in each downlink beam 27 onto a carrier ofapproximately 2.5 MHz bandwidth.

Each uplink PRC channel can be routed to all, some or none of thedownlink beams 27. The order and placement of PRCs in a downlink beam isprogrammable and selectable from a telemetry, range and control (TRC)facility 24 (FIG. 1). Each polyphase demultiplexer and demodulator 122receives the individual FDMA uplink signals in groups of 48 contiguouschannels and generates a single analog signal on which the data of the48 FDMA signals is time multiplexed, and performs a high speeddemodulation of the serial data as described in further detail below inconnection with FIG. 6. Six of these polyphase demultiplexer anddemodulators 122 operate in parallel to process 288 FDMA signals. Arouting switch and modulator 124 selectively directs individual channelsof the six serial data streams into all, some or none of the downlinksignals 27 and further modulates and up-converts the three downlink TDMsignals 27. Three traveling wave tube amplifiers (TWTA) 126 individuallyamplify the three downlink signals, which are radiated to the earth byL-band transmit antennas 128.

The satellite 25 also contains three transparent payloads, eachcomprising a demultiplexer and down-converter 130 and an amplifier group132 configured in a conventional "bent pipe" signal path which convertsthe frequency of input signals for retransmission. Thus, each satellite25 in the system 10 is preferably equipped with two types ofcommunication payloads. The first type of on-board processing payload isdescribed with reference to FIGS. 5, 6 and 7. The second type ofcommunication payload is the transparent payload which converts uplinkTDM carriers from frequency locations in the uplink X-band spectrum tofrequency locations in the L-band downlink spectrum. The transmitted TDMstream for the transparent payload is assembled at a broadcast station23, sent to the satellite 25, received and frequency converted to adownlink frequency location using module 130, amplified by a TWTA inmodule 132 and transmitted to one of the beams. To a radio receiver 29,the TDM signals appear identical whether they are from the on-boardprocessing payload indicated at 121 or the transparent payload indicatedat 133. The carrier frequency locations of each type of payload 121 and133 are spaced on separate grids of 920 kHz spacing which are interlacedbetween one another in a bisected manner so that the carrier locationsof a mix of signals from both types of payloads 121 and 133 are on 460kHz spacings.

The on-board demultiplexer and demodulator 122 will now be described infurther detail with reference to FIG. 6. As shown in FIG. 6, SCPC/FDMAcarriers, each of which is designated with reference numeral 136, areassigned to groups of 48 channels. One group 138 is shown in FIG. 6 forillustrative purposes. The carriers 136 are spaced on a grid of centerfrequencies separated by 38 kHz. This spacing determines designparameters of the polyphase demultiplexers. For each satellite 25,preferably 288 uplink PRC SCPC/FDMA carriers can be received from anumber of broadcast stations 23. Six polyphase demultiplexers anddemodulators 122 are therefore preferably used. An on-board processor116 accepts these PRC SCPC/FDMA uplink carriers 136 and converts theminto three downlink TDM carriers, each carrying 96 of the PRCs in 96time slots.

The 288 carriers are received by an uplink global beam antenna 118 andeach group of 48 channels is frequency converted to an intermediatefrequency (IF) which is then filtered to select a frequency bandoccupied by that particular group 138. This processing takes places inthe receiver 120. The filtered signal is then supplied to ananalog-to-digital (A/D) converter 140 before being supplied as an inputto a polyphase demultiplexer 144. The demultiplexer 144 separates the 48SCPC/FDMA channels 138 into a time division multiplexed analog signalstream comprising QPSK modulated symbols that sequentially present thecontent of each of 48 SCPC/FDMA channels at the output of thedemultiplexer 144. This TDM analog signal stream is routed to adigitally implemented QPSK demodulator and differential decoder 146. TheQPSK demodulator and differential decoder 146 sequentially demodulatesthe QPSK modulated symbols into digital baseband bits. Demodulationprocessing requires symbol timing and carrier recovery. Since themodulation is QPSK, baseband symbols containing two-bits each arerecovered for each carrier symbol. The demultiplexer 144 and demodulatorand decoder 146 will hereinafter be referred to as ademultiplexer/demodulator (D/D) 148. The D/D is preferably accomplishedusing high speed digital technology using the known Polyphase techniqueto demultiplex the uplink carriers 21. The QPSK demodulator ispreferably a serially-shared, digitally-implemented demodulator forrecovering the baseband two-bit symbols. The recovered symbols 114 fromeach PRC carrier 110 are subsequently differentially decoded to recoverthe original PRC symbols 108 applied at the input encoders, that is, thechannel distributors 82 and 98 in FIG. 3, at the broadcast station 23.The satellite 25 payload preferably comprises six digitally implemented,48 carrier D/Ds 148. In addition, two spare D/Ds 148 are provided in thesatellite payload to replace any failed processing units.

With continued reference to FIG. 6, the processor 116 is programmed inaccordance with a software module indicated at 150 to perform asynchronization and rate alignment function on the time divisionmultiplexed symbol stream generated at the output of the QPSKdemodulator and differential decoder 146. The software and hardwarecomponents (e.g., digital memory buffers and oscillators) of the ratealignment module 150 in FIG. 6 are described in more detail withreference to FIG. 7. The rate alignment module 150 compensates for clockrate differences between the on-board clock 152 and that of the symbolscarried on the individual uplink PRC carriers 138 received at thesatellite 25. The clock rates differ because of different clock rates atdifferent broadcast stations 23, and different Doppler rates fromdifferent locations caused by motion of the satellite 25. Clock ratedifferences attributed to the broadcast stations 23 can originate inclocks at a broadcast station itself or in remote clocks, the rates ofwhich are transferred over terrestrial links between a broadcast studioand a broadcast station 23.

The rate alignment module 150 adds or removes a "0" value symbol, ordoes neither operation in the PRC header portion 112 of each 432 msrecovered frame 100. A "0" value symbol is a symbol that consists of abit value 0 on both the I and Q channels of the QPSK-modulated symbol.The PRC header 112 comprises 48 symbols under normal operatingconditions and consists of an initial symbol of "0" value, followed by47 other symbols. When the symbol times of the uplink clock, which isrecovered by the QPSK demodulator 146 along with the uplink carrierfrequency, and those of the on-board clock 152 are synchronized, nochange is made to the PRC preamble 112 for that particular PRC 110. Whenthe arriving uplink symbols have a timing that lags behind the on-boardclock 152 by one symbol, a "0" symbol is added to the start of the PRCpreamble 112 for the PRC currently being processed, yielding a length of49 symbols. When the arriving uplink symbols have a timing that leadsthe on-board clock 152 by one symbol, a "0" symbol is deleted at thestart of the PRC preamble 112 of the current PRC being processed,yielding a length of 47 symbols.

As stated previously, the input signal to the rate alignment module 150comprises the stream of the recovered baseband two-bit symbols for eachreceived uplink PRC at their individual original symbol rates. There are288 such streams issued from the D/D 148 corresponding to each of thesix active processors 116. The action involving only one D/D 148 and onerate alignment module 150 is described, although it is to be understoodthat the other five active processors 116 on the satellite performsimilar functions.

To rate align uplink PRC symbols to the on-board clock 152, three stepsare performed. First, the symbols are grouped in terms of their original8208 two-bit symbol PRC frames 110 in each buffer 149 and 151 of aping-pong buffer 153. This requires correlation of the PRC header 112(which contains a 47 symbol unique word) with a local stored copy of theunique word in correlators indicated at 155 to locate the symbols in abuffer. Second, the number of on-board clock 152 ticks betweencorrelation spikes is determined and used to adjust the length of thePRC header 112 to compensate for the rate difference. Third, the PRCframe, with its modified header, is clocked at the on-board rate intoits appropriate location in a switching and routing memory device 156(FIG. 8).

PRC symbols enter the ping-pong buffer pair 153 at the left. Theping-pong action causes one buffer 149 or 151 to fill at the uplinkclock rate, and the other buffer to simultaneously empty at the on-boardclock rate. The roles reverse from one frame to the next and causecontinuous flow between input and output of the buffer 149 and 151.Newly arriving symbols are written to the buffer 149 or 151 to whichthey happen to be connected. Writing continues to fill the buffer 149 or151 until the correlation spike occurs. Writing then stops, and theinput and output switches 161 and 163 switch to the reverse state. Thiscaptures an uplink PRC frame so that its 48 header symbols reside in the48 symbol slots with one slot left unfilled at the output end of thebuffer and the 8160 data symbols fill the first 8160 slots. The contentsof the subject buffer are immediately read to the output thereof at theon-board clock rate. The number of symbols read out are such that thePRC header contains 47, 48 or 49 symbols. A "0" value symbol is removedor added at the start of the PRC header to make this adjustment. Theheader length 112 is controlled by a signal coming from a frame symbolcounter 159 which counts the number of on-board clock rate symbols thatwill fall in a PRC frame period to determine the header length. Theping-pong action alternates the roles of the buffers.

To perform the count, the frame correlation spikes coming from thebuffer correlators 155, as PRC frames fill the buffers 149 and 151, aresmoothed by a synch pulse oscillator (SPC) 157. The smoothed sync pulsesare used to count the number of symbol epochs per frame. The number willbe 8207, 8208 or 8209 indicating whether the PRC header should be 47, 48or 49 symbols long, respectively. This information causes the propernumber of symbols to come from the frame buffers to maintain symbol flowsynchronously with the on-board clock and independently of earthterminal origin.

For the rate differences anticipated over the system 10, the run timesbetween preamble 112 modifications are relatively long. For instance,clock rate differences of 10⁻⁶ will elicit PRC preamble corrections onthe average of one every 123 PRC frames. The resulting rate adjustmentscause the symbol rates of the PRCs 110 to be precisely synchronized tothe on-board clock 152. This allows routing of the baseband bit symbolsto the proper locations in a TDM frame. The synchronized PRCs areindicated generally at 154 in FIG. 6. The on-board routing and switchingof these PRCs 154 into TDM frames will now be described with referenceto FIG. 8.

FIG. 6 illustrates PRC processing by a single D/D 148. Similarprocessing is performed by the other five active D/Ds on-board thesatellite. The PRCs emanating from each of the six D/Ds 148, having beensynchronized and aligned, occur in a serial stream having a symbol rateof 48×19,000 which equals 912,000 symbols per second for each D/D 148.The serial stream from each D/D 148 can be demultiplexed into 48parallel PRC streams having rates of 19,000 symbols per second, as shownin FIG. 7. The aggregate of the PRC streams coming from all six D/Ds 148on-board the satellite 25 is 288, with each D/D 148 carrying 19,000sym/s streams. The symbols therefore have epochs or periods of 1/19,000seconds which equals approximately 52.63 microseconds duration.

As shown in FIG. 8, 288 symbols are present at the outputs of the sixD/Ds 148a, 148b, 148c, 148d, 148e and 148f for every uplink PRC symbolepoch. Once each PRC symbol epoch, 288 symbol values are written into aswitching and routing memory 156. The contents of the buffer 156 areread into three downlink TDM frame assemblers 160, 162 and 164. Using arouting and switching component designated as 172, the contents of eachof the 288 memory locations are read in terms of 2622 sets of 96 symbolsto each of the three TDM frames in assemblers 160, 162 and 164 in anepoch of 136.8 ms which occurs once every TDM frame period or 138 ms.The scan rate or 136.8/2622 is therefore faster than the duration of asymbol. The routing switch and modulator 124 comprises a ping-pongmemory configuration indicated generally at 156 and comprising buffers156a and 156b, respectively. The 288 uplink PRCs indicated at 154 aresupplied as input to the routing switch and modulator 124. The symbolsof each PRC occur at a rate of 19,000 symbols per second corrected tothe on-board clock 152 timing. The PRC symbols are written in parallelat the 19,000 Hz clock rate into 288 positions in the ping-pong memory156a or 156bserving as the input. At the same time, the memory servingas the output 156b or 156a, respectively, is reading the symbols storedin the previous frame into the three TDM frames at a read rate of 3×1.84Mhz. This latter rate is sufficient to allow the simultaneous generationof the three TDM parallel streams, one directed to each of three beams.Routing of the symbols to their assigned beam is controlled by a symbolrouting switch 172. This switch can route a symbol to any one, two orthree of the TDM streams. Each TDM stream occurs at a rate of 1.84Msym/s. The output memory is clocked for an interval of 136.8 ms andpauses for 1.2 ms to allow insertion of the 96 symbol MFP and 2112symbol TSCC. Note that for every symbol that is read into more than oneTDM stream, there is an off-setting uplink FDM PRC channel that is notused and is skipped. The ping-pong memory buffers 156a and 156b exchangeroles from frame to frame via the switch components 158a and 158b.

With continued reference to FIG. 8, sets of 96 symbols are transferredto 2622 corresponding slots in each TDM frame. The corresponding symbols(i.e., the ith symbols) for all 96 uplink PRCs are grouped together inthe same TDM frame slot as illustrated by the slot 166 for symbol 1. Thecontents of the 2622 slots of each TDM frame are scrambled by adding apseudorandom bit pattern to the entire 136.8 ms epoch. In addition, a1.2 ms epoch is appended at the start of each TDM frame to insert amaster frame preamble (MFP) of 96 symbols and a TSCC of 2112 symbols, asindicated at 168 and 170, respectively. The sum of the 2622 time slots,each carrying 96 symbols, and the symbols for the MFP and TSCC is253,920 symbols per TDM frame, resulting in a downlink symbol rate of1.84 Msym/s.

The routing of the PRC symbols between the outputs of the six D/Ds 148A,148B, 148C, 148D, 148E and 148F and the inputs to the TDM frameassemblers 160, 162 and 164 is controlled by an on-board switchingsequence unit 172 which stores instructions sent to it over a commandlink from the SCC 238 (FIG. 12) from the ground. Each symbol originatingfrom a selected uplink PRC symbol stream can be routed to a time slot ina TDM frame to be transmitted to a desired destination beam 27. Themethod of routing is independent of the relationships between the timeof occurrence of symbols in various uplink PRCs and the occurrence ofsymbols in the downlink TDM streams. This reduces the complexity of thesatellite 25 payload. Further, a symbol originating from a selecteduplink PRC can be routed to two or three destination beams via theswitch 158.

Radio Receiver Operation

A radio receiver 29 for use in the system 10 will now be described withreference to FIG. 9. The radio receiver 29 comprises an radio frequency(RF) section 176 having an antenna 178 for L-band electromagnetic wavereception, and prefiltering to select the operating band of the receiver(e.g., 1452 to 1492 MHz). The RF section 176 further comprises a lownoise amplifier 180 which is capable of amplifying the receive signalwith minimum self-introduced noise and of withstanding interferencesignals that may come from another service sharing the operating band ofthe receiver 29. A mixer 182 is provided to down-convert the receivedspectrum to an intermediate frequency (IF). A high performance IF filter184 selects the desired TDM carrier bandwidth from the output of themixer 182 and a local oscillator synthesizer 186, which generates themixing input frequencies needed to down-convert the desired signal tothe center of the IF filter. The TDM carriers are located on centerfrequencies spaced on a grid having 460 kHz separations. The bandwidthof the IF filter 184 is approximately 2.5 MHz. The separation betweencarriers is preferably at least seven or eight spaces or approximately3.3 MHz. The RF section 176 is designed to select the desired TDMcarrier bandwidth with a minimum of internally-generated interferenceand distortion and to reject unwanted carriers that can occur in theoperating band from 152 to 192 MHz. In most areas of the world, thelevels of unwanted signals are nominal, and typically the ratios ofunwanted signals to desired signals of 30 to 40 dB provides sufficientprotection. In some areas, operations near high power transmitters(e.g., in the vicinity of terrestrial microwave transmitters for publicswitched telephone networks or other broadcast audio services) requiresa front end design capable of better protection ratios. The desired TDMcarrier bandwidth retrieved from the downlink signal using the RFsection 176 is provided to an A/D converter 188 and then to a QPSKdemodulator 190. The QPSK demodulator 190 is designed to recover the TDMbit stream transmitted from satellite 25, that is, via the on-boardprocessor payload 121 or the on-board transparent payload 133, on aselected carrier frequency.

The QPSK demodulator 190 is preferably implemented by first convertingthe IF signal from the RF section 176 into a digital representationusing the A/D converter 188, and then implementing the QPSK using aknown digital processing method. Demodulation preferably uses symboltiming and carrier frequency recover and decision circuits which sampleand decode the symbols of the QPSK modulated signal into the basebandTDM bit stream.

The A/D converter 188 and QPSK demodulator 190 are preferably providedon a channel recovery chip 187 for recovering the broadcast channeldigital baseband signal from the IF signals recovered by the RF/IFcircuit board 176. The channel recovery circuit 187 comprises a TDMsynchronizer and predictor module 192, a TDM demultiplexer 194, a PRCsynchronizer alignment and multiplexer 196, the operations of which willbe described in further detail in connection with FIG. 10. The TDM bitstream at the output of the QPSK demodulator 190 is provided to a MFPsynchronization correlator 200 in the TDM synchronizer and predictormodule 192. The correlator 200 compares the bits of the received streamto a stored pattern. When no signal has previously been present at thereceiver, the correlator 200 first enters a search mode in which itsearches for the desired MFP correlation pattern without any time gatingor aperture limitation applied to its output. When the correlatordiscovers a correlation event, it enters a mode wherein a gate opens ata time interval in which a next correlation event is anticipated. If acorrelation event occurs again within the predicted time gate epoch, thetime gating process is repeated. If correlation occurs for fiveconsecutive time frames, for example, synchronization is declared tohave been determined in accordance with the software. Thesynchronization threshold, however, can be changed. If correlation hasnot occurred for the minimum number of consecutive time frames to reachthe synchronization threshold, the correlator continues to search forthe correlation pattern.

Assuming that synchronization has occurred, the correlator enters asynchronization mode in which it adjusts its parameters to maximizeprobability of continued synchronization lock. If correlation is lost,the correlator enters a special predictor mode in which it continues toretain synchronization by prediction of the arrival of the nextcorrelation event. For short signal dropouts (e.g., for as many as tenseconds), the correlator can maintain sufficiently accuratesynchronization to achieve virtually instantaneous recovery when thesignal returns. Such rapid recovery is advantageous because it isimportant for mobile reception conditions. If, after a specified period,correlation is not reestablished, the correlator 200 returns to thesearch mode. Upon synchronization to the MFP of the TDM frame, the TSCCcan be recovered by the TDM demultiplexer 194 (block 202 in FIG. 10).The TSCC contains information identifying the program providers carriedin the TDM frame and in which locations of the 96 PRCs each programprovider's channel can be found. Before any PRCs can be demultiplexedfrom the TDM frame, the portion of the TDM frame carrying the PRCsymbols is preferably descrambled. This is done by adding the samescrambling pattern at the receiver 29 that was added to the PRC portionof the TDM frame bit stream on-board the satellite 25. This scramblingpattern is synchronized by the TDM frame MFP.

The symbols of the PRCs are not grouped contiguously in the TDM frame,but are spread over the frame. There are 2622 sets of symbols containedin the PRC portion of the TDM frame. In each set, there is one symbolfor each PRC in a position which is numbered in ascending order from 1to 96. Thus, all symbols belonging to PRC 1 are in the first position ofall 2622 sets. Symbols belonging to PRC 2 are in the second position ofall 2622 sets, and so on, as shown in block 204. This arrangement fornumbering and locating the symbols of the PRCs in the TDM frame, inaccordance with the present invention, minimizes the size of the memoryfor performing the switching and routing on-board the satellite and fordemultiplexing in the receiver. As shown in FIG. 9, the TSCC isrecovered from the TDM demultiplexer 194 and provided to the controller220 at the receiver 29 to recover the n PRCs for a particular broadcastchannel. The symbols of the n PRCs associated with that broadcastchannel are extracted from the unscrambled TDM frame time slot locationsidentified in the TSCC. This association is performed by a controllercontained in the radio and is indicated generally at 205 in Fig. 10. Thecontroller 220 accepts a broadcast selection identified by the radiooperator, combines this selection with the PRC information contained inthe TSCC, and extracts and reorders the symbols of the PRCs from the TDMframe to restore the n PRCs.

With reference to blocks 196 and 206, respectively, in FIGS. 9 and 10,the symbols of each of the n PRCs associated with a broadcast channelselected by the radio operator are remultiplexed into an FEC-codedbroadcast channel (BC) format. Before the remultiplexing isaccomplished, the n PRCs of a broadcast channel are realigned.Realignment is useful because reclocking of symbol timing encountered inmultiplexing, demultiplexing and on-board rate alignment in passage overthe end-to-end link in system 10 can introduce a shift of as many asfour symbols in the relative alignment of the recovered PRC frames. Eachof the n PRCs of a broadcast channel has a 48 symbol preamble, followedby 8160 coded PRC symbols. To recombine these n PRCs into the broadcastchannel, synchronization is performed to the 47, 48 or 49 symbol headerof each of the PRCs. The length of the header depends on the timingalignment performed on the uplink PRCs on the satellite 25.Synchronization is accomplished using a preamble correlator operating onthe 47 most recently received symbols of the PRC header for each of then PRCs. The preamble correlator detects incidents of correlation andemits a single symbol duration correlation spike. Based on the relativetime of occurrence of the correlation spikes for the n PRCs associatedwith the broadcast channel, and operating in conjunction with alignmentbuffers having a width of four symbols, the symbol content of the n PRCs(e.g., as indicated at 207) can be precisely aligned and remultiplexedto recover the FEC-coded broadcast channel (e.g., as indicated at 209).Remultiplexing of the n PRCs to reform the FEC-coded broadcast channelpreferably requires that the symbol spreading procedure used at thebroadcast station 23 for demultiplexing the FEC-coded broadcast channelinto the PRCs be performed in the reverse order, as indicated in blocks206 and 208 of FIG. 10.

FIG. 11 illustrates how a broadcast channel, comprising four PRCs, forexample, is recovered at the receiver (block 196 in Fig. 9). At theleft, four demodulated PRCs are shown arriving. Due to reclockingvariations, and different time delays encountered from the broadcaststation through the satellite to the radio, up to four symbols ofrelative offset can occur among the n PRCs constituting a broadcastchannel. The first step in recovery is to realign the symbol content ofthese PRCS. This is done by a set of FIFO buffers each having a lengthequal to the range of variation. Each PRC has its own buffer 222. EachPRC is first supplied to a PRC header correlator 226 that determines theinstant of arrival. The arrival instants are shown by a correlationspike 224 for each of the four PRCs in the illustration. Writing (W)starts into each buffer 222 immediately following the instant ofcorrelation and continues thereafter until the end of the frame. Toalign the symbols to the PRCs, reading (R) from all of the buffers 222starts at the instant of the last correlation event. This causes thesymbols of all PRCs to be synchronously read out in parallel at thebuffer 222 outputs (block 206). The realigned symbols 228 are nextmultiplexed via a multiplexer 230 into a single serial stream that isthe recovered coded broadcast channel 232 (block 208). Due to on-boardclock 152 rate alignment, the length of the PRC header may be 47, 48 or49 symbols long. This variation is eliminated in the correlator 226 byusing only the last 47 symbols to arrive to detect the correlationevent. These 47 symbols are specially selected to yield optimumcorrelation detection.

With reference to block 198 and 210 of FIGS. 9 and 10 respectively, theFEC-coded broadcast channel is subsequently provided to the FECprocessing module 210. Most of the errors encountered in transmissionbetween the location of the coders and the decoders is corrected by FECprocessing. FEC processing preferably employs a Viterbi Trellis Decoder,followed by deinterleaving and then a Reed Solomon decoder. FECprocessing recovers the original broadcast channel comprising n×16 kbpschannel increments and its n×224 bit SCH (block 212).

The n×16 kbps segment of the broadcast channel is provided to a decodersuch as MPEG 2.5 Layer 3 source decoder 214 for conversion back to audiosignals. Thus, receiver processing is available using a low cost radiofor broadcast channel reception from satellites. Since the transmissionsof the broadcast programs via satellites 25 is digital, a number ofother services are supported by the system 10 which are also expressedin digital format. As stated previously, the SCH contained in thebroadcast channels provides a control channel for a wide variety offuture service options. Thus, chip sets can be produced to implementthese service options by making the entire TDM bit stream and its rawdemodulated format, the demultiplexed TSCC information bits, and therecovered error corrected broadcast channel available. Radio receivers29 can also be provided with an identification code for uniquelyaddressing each radio. The code can be accessed by means of bits carriedin a channel of the SCH of the broadcast channel. For mobile operationusing the radio receiver 29 in accordance with the present invention,the radio is configured to predict and recover substantiallyinstantaneously the locations of MFP correlation spikes to an accuracyof 1/4th symbol for intervals of as many as ten seconds. A symbol timinglocal oscillator having a short time accuracy of better than one partper 100,000,000 is preferably installed in the radio receiver,particularly for a hand-held radio 29b.

System for Managing Satellite and Broadcast Stations

As stated previously, the system 10 can comprise one or a plurality ofsatellites 25. FIG. 12 depicts three satellites 25a, 25b and 25c forillustrative purposes. A system 10 having several satellites preferablycomprises a plurality of TCR stations 24a, 24b, 24c, 24d and 24e locatedsuch that each satellite 25a, 25b and 25c is in line of sight of two TCRstations. The TCR stations referred to generally with reference numeral24 are controlled by a regional broadcast control facility (RBCF) 238a,238b or 238c. Each RBCF 238a, 238b and 238c comprises a satellitecontrol center (SCC) 236a, 236b and 236c, a mission control center (MCC)240a, 240b and 240c, and a broadcast control center (BCC) 244a, 244b and244c, respectively. Each SCC controls the satellite bus and thecommunications payload and is where a space segment command and controlcomputer and manpower resources are located. The facility is preferablymanned 24 hours a day by a number of technicians trained in in-orbitsatellite command and control. The SCCs 236a, 236b and 236c monitor theon-board components and essentially operate the corresponding satellite25a, 25b and 25c. Each TCR station 24 is preferably connected directlyto a corresponding SCC 236a, 236b or 236c by full-time, dual redundantPSTN circuits.

In each of the regions serviced by the satellites 25a, 25b and 25c, thecorresponding RBCF 238a, 238b and 238c reserves broadcast channels foraudio, data, video image services, assigns space segment channel routingvia the mission control center (MCC) 240a, 240b, 240c, validates thedelivery of the service, which is information required to bill abroadcast service provider, and bills the service provider.

Each MCC is configured to program the assignment of the space segmentchannels comprising uplink PRC frequency and downlink PRC TDM slotassignments. Each MCC performs both dynamic and static control. Dynamiccontrol involves controlling time windows for assignments, that is,assigning space segment usage on a monthly, weekly and daily basis.Static control involves space segment assignments that do not vary on amonthly, weekly and daily basis. A sales office, which has personnel forselling space segment capacity at the corresponding RBCF, provides theMCC with data indicating available capacity and instructions to seizecapacity that has been sold. The MCC generates an overall plan foroccupying the time and frequency space of the system 10. The plan isthen converted into instructions for the on-board routing switch 172 andis sent to the SCC for transmission to the satellite. The plan can beupdated and transmitted to the satellite preferably once every 12 hours.The MCC 240a, 240b and 240c also monitors the satellite TDM signalsreceived by corresponding channel system monitor equipment (CSME) 242a,242b and 242c. CSME stations verify that broadcast stations 23 aredelivering broadcast channels within specifications.

Each BCC 244a, 244b and 244c monitors the broadcast earth stations 23 inits region for proper operation within m selected frequency, power andantenna pointing tolerances. The BCCs can also connect withcorresponding broadcast stations to command malfunctioning stationsoff-the-air. A central facility 246 is preferably provided for technicalsupport services and back-up operations for each of the SCCs.

While certain advantageous embodiments have been chosen to illustratethe invention, it will be understood by those skilled in the art thatvarious changes and modifications can be made therein without departingfrom the scope of the invention as defined in the appended claims.

What is claimed is:
 1. A method of receiving one of a plurality ofbroadcast channels transmitted via downlink signals comprising primerate channels from a satellite comprising the steps of:demodulating saiddownlink signals into a baseband time division multiplexed bit streamcomprising frames generated by said satellite, each of said framescomprising a plurality of time slots, each of said time slots comprisinga set of symbols, each symbol in said set of symbols corresponding to arespective one of said prime rate channels occupying a similar symbolposition in each of said time slots; locating said frames in said bitstream using a master frame preamble inserted in said bit stream by saidsatellite; retrieving from said set of symbols in each of said timeslots of at least one of said frames the symbols that correspond to atleast one of said prime rate channels; remultiplexing said symbolscorresponding to said at least one of said prime rate channels torecover a broadcast channel corresponding thereto and as originallytransmitted to said satellite; and extracting a service control headerfrom said broadcast channel.
 2. A method as claimed in claim 1, whereinsaid retrieving step comprises the steps of:locating a time slot controlchannel inserted in said bit stream by said satellite, said controlchannel indicating which of said time slots contain said symbolscorresponding to each of said prime rate channels; and extracting saidsymbols corresponding to one of said prime rate channels selected by ausing said control channel.
 3. The method as claimed in claim 1, furthercomprising the step of determining if said service control headercomprises an identification code inserted in said broadcast channel by abroadcast station prior to transmission to said satellite for uniquelyaddressing a radio receiver.
 4. The method as claimed in claim 1,further comprising the steps of:determining if said service controlheader comprises control data; and operating a radio receiver to performat least one of a plurality of functions depending on said control data,said plurality of functions comprising operating said radio receiver ina selected reception mode to provide a selected multimedia service, todisplay data, to display an image, and to decrypt data using adecryption key provided in said service control header.
 5. A radioreceiver for receiving one of a plurality of prime rate channelstransmitted via downlink signals from a satellite comprising:a radiofrequency device for receiving said downlink signals; a channel recoverydevice for recovering said prime rate channels from said downlinksignals by demodulating said downlink signals into a baseband timedivision multiplexed bit stream comprising frames generated by saidsatellite, each of said frames comprising a plurality of time slots,each of said time slots comprising a set of symbols, each symbol in saidset of symbols corresponding to a respective one of said prime ratechannels occupying a similar symbol position in each of said time slots,locating said frames in said bit stream using a master frame preambleinserted in said bit stream by said satellite, retrieving from said setof symbols in each of said time slots of at least one of said frames thesymbols that correspond to at least one of said prime rate channels,remultiplexing said symbols corresponding to said at least one of saidprime rate channels to recover a broadcast channel corresponding theretoand as originally transmitted to said satellite, and extracting aservice control header from said broadcast channel; and a controller,said controller being operable to receive said service control headerfrom said channel recovery device and control said radio receiver toperform a plurality of functions comprising operating said radioreceiver in a selected reception mode to provide a selected multimediaservice, to display data, to display an image, to decrypt data using adecryption key provided in said service control header, and to respondto an identification code provided in said service control header touniquely address said radio receiver.